1. Field of the Invention
The present invention relates to a hot melt ink utilizable for an ink jet printer, in particular to a hot melt ink which exists in a solid state at a room temperature (ordinary temperature) and is used in an ink jet printer after being thermally melted over the room temperature when printing is conducted by the ink jet printer.
2. Description of Related Art
In general, an ink jet printer has various advantages in printing because of being noiseless, high speed printing, high quality printing and ability of color printing. On the other hand, since the ink jet printer conducts printing by directly adhering onto a print sheet ink droplets jetted from nozzles, printing quality or drying time of the ink droplets is influenced by quality of the print sheet, as a result, there is a problem that printing quality of the ink jet printer remarkably goes down when printing is conducted onto a print sheet with low quality.
In order to dissolve the above problem, it has been conventionally proposed an ink jet printer which can print images with ink dots having uniform diameters onto the print sheet in spite of quality of the print sheet, by using a hot melt type ink which has a melting point higher than the room temperature. As ink components of the hot melt ink, it is known an ink including a natural wax disclosed in the specification of U.S. Pat. No. 4,390,369 (Japanese Patent Application Laid Open No. Sho 58-108,271), an ink including stearic acid disclosed in the specification of U.S. Pat. No. 4,758,276 (Japanese Patent Application Laid Open No. SHo 59-22,973), and an ink which includes acid or alcohol of carbon number 20-24 and acid or alcohol of carbon number 14-19 and includes keton a having relatively high melting point and a dye as colorant, disclosed in the specification of U.S Pat. No. 4,659,383 (Japanese Patent Application Laid Open No. Sho 61-83,268). Further, it is disclosed in U.S. Pat. No. 4,659,383 and Japanese Patent Application after substantive examination, Laid Open No. Hei 2-18,710, a hot melt ink in which a solid pigment is dispersed in wax having a melting point higher than 65.degree. C., or fatty acid or alcohol of carbon number 18-24.
In Japanese Patent Application Laid Open No. Hei 5-125,316, it is disclosed hot melt ink components which include a natural wax having penetration grade less than 2, the penetration grade being measured by a penetration testing apparatus and appreciated based on a depth that a needle penetrates under a predetermined condition according to Japanese Industrial Standard (JIS) K2207, K2220, K2235, dye dissolving agent with a melting point higher than 80.degree. C. and dye. By utilizing such hot melt ink, it can prevent inferiority of printing quality occurring due to blurring generated when the ink is re-melted during ink fixation by heat or pressure, or due to offset generated when the ink is adhered to a pressure roller.
However, the above mentioned hot melt ink cannot be sufficiently soaked into the print sheet, thereby fixation property of the hot melt ink is not enough. On the contrary, the conventional hot melt ink tends to soak into the print sheet too easily, as a result, the ink penetrates to the rear surface of the print sheet. Further, among the conventional hot melt inks, there exists an ink which is solidified while it does not sufficiently spread on the printing surface of the print sheet after ink droplets thereof reach onto the printing surface. In such case of the hot melt ink, it cannot be obtained sufficient printing density (quality), since gaps between ink dots are outstanding and surfaces of the ink dots become hemispherical rather than flat. Contrarily, in case that the hot melt ink tends to soak into the print sheet too easily, there is a problem that printing density on the printing surface is made thin.
As concerns the fixation of the ink and the printing density, in Japanese Patent Application Laid Open No. Hei 5-125,316 mentioned above, it is able to improve both the fixation and the printing density by fixing the ink dots through heating or pressure roller. But, though the fixation property of the ink and the printing density will be improved to some extent by using the above methods if the ink cannot be sufficiently soaked into the print sheet, a fixing device such as a heater or a pressure roller has to be additionally installed in the ink jet printer. Thereby, cost of the ink jet printer necessarily increases. Further, it cannot take measures against the fixation property or the printing density if it is used the ink which is soaked into the print sheet too easily.